This invention relates to phase control device. More particularly, the invention relates to a received signal phase control device which is employed as the demodulation circuit of a compatible quadrature AM stereophonic signal, that is, one which is compatible with a stereophonic signal receiver and a monaural signal receiver.
One of the stereophonic signals in a compatible quadrature PM system is an AM stereophonic signal. Such a stereophonic signal e.sub.i can be represented by the following equation (1): EQU e.sub.1 =[{1+k(L(t)+R(t))}.multidot.cos .omega..sub.i t+k{L(t)-R(t)}.multidot.cos (.omega..sub.i t+.pi./2)].multidot.cos .phi., (1)
where .phi.=tan.sup.-1 k{L(t)-R(t)}/[1+k{L(t)+R(t)}], L(t) and R(t) are the left and right channels signals, .omega..sub.i is the angular frequency of the carrier signal, and k is the modulation factor.
That is, the compatible quandrature AM stereophonic signal is produced by synthesizing a signal which is obtained by modulating the amplitude of the carrier signal cos .omega..sub.i t with a signal corresponding to the sum of the two channel signals and another signal which is obtained by modulating the carrier signal cos (.omega..sub.i t+.lambda./2), which is shifted by 90.degree. in phase with respect to the aforementioned carrier signal, with a signal corresponding to the difference between the two channel signals. The synthesized signal is transmitted with the level being modulated with cos .phi. as indicated in the equation (1) so that the stereophonic signal can be received by a monaural signal receiver.
A second type of stereophonic signal can be represented by the following equation: ##EQU1##
A stereophonic signal of the form indicated by the equation (2) can be demodulated by a circuit as shown in FIG. 1. A received input signal is mixed with a local oscillation signal e.sub.L produced by a local oscillator in a mixer 2 as a result of which an intermediate frequency signal e.sub.i is obtained. The intermediate frequency signal, after being amplified by an IF amplifier 3, is applied to a divider 4.
In the divider 4, the cos .phi. component is removed from the intermediate frequency signal e.sub.i and the resulting signal is applied to a quadrature stereophonic demodulation circuit 5. In this demodulation circuit 5, a differential circuit-type product demodulator is employed to produce products of the output of the divider 4 and signal components cos (.omega..sub.i t+.pi./4) and cos (.omega..sub.i t-.pi./4), the product output signals being the signal components L(t) and R(t).
In order to produce the signal components cos .phi., cos (.omega..sub.i t+.pi./4) and cos (.omega..sub.i t-.pi./4) for use in this demodulation circuit, a phase-locked loop (PLL) circuit 10 and phase shifters are used. That is, the intermediate frequency signal e.sub.i, after being converted into a square-wave signal by a limiter 6, is applied to one input of a phase comparator 7. The output of the phase comparator 7 is applied through a low-pass filter (LPF) 8 to a DC amplifier 9 where it is amplified and is then applied as a control voltage to a voltage-controlled oscillator (VCO) 11. The output e.sub.o of the voltage-controlled oscillator 11 is applied to the other input of the phase comparator 7 and an error voltage V.sub.1 corresponding to the frequency of the aforementioned input e.sub.i and the phase difference between the two inputs is thereby formed at the output of the phase comparator 7,
The output e.sub.o of the voltage-controlled oscillator 11 is applied to one input terminal of an in-phase detector 13 after being phase shifted 90.degree. by a .pi./2 phase shifter 12. The intermediate frequency signal e.sub.i is applied to the other input terminal of the in-phase detector 13 from which the latter produces the component cos .phi. which is applied to the divider 4.
The output of the .pi./2 phase shifter 12 is shifted in phase by .+-.45.degree. by a .pi./4 phase shifter 14 and a .pi./4 phase shifter 15 so that the components cos (.omega..sub.i t+.pi./4) and cos (.omega..sub.i t-.pi./4) are produced thereby after which they are applied to the quadrature demodulation circuit 5.
In the case where the phase comparator 7 in the PLL circuit 10 provides the output voltage V.sub.1 proportional to the cosine of the phase difference between the two input signals, the phase difference .DELTA..PHI..sub.e between the two input signals can be represented by the following equation (3): EQU .DELTA..PHI..sub.e =cos.sup.-1 .DELTA..omega./Kd, (3)
where Kd is the loop gain of the PLL circuit, and .DELTA..omega. is the difference between the angular frequency .omega..sub.i of the input signal e.sub.i and the free-running frequency .omega..sub.o of the voltage-controlled oscillator 11.
Thus, as is clear from the equation (3), when .DELTA..omega. is zero, that is, when the input signal e.sub.i is equal to the free-running frequency of the voltage-controlled oscillator 11, .DELTA..PHI..sub.e is 90.degree. and the phase of the output e.sub.o of the voltage-controlled oscillator 11 is shifted by 90.degree. from the phase of the input signal e.sub.i. The signal components cos .phi., cos (.omega..sub.i t+.pi./4) and cos (.omega..sub.i t-.pi./4) obtained with use of the signal e.sub.o have regular phases and therefore correct quadrature stereophonic demodulation can be performed.
However, if for instance the frequency of the local oscillation signal e.sub.L is slightly shifted due to temperature drift or the like, the frequency of the intermediate frequency signal e.sub.i also shifts as a result of which the value .DELTA..omega. in the equation (3) will not be zero. In this case, the phase difference .DELTA..PHI..sub.e between the signal e.sub.i and the output signal e.sub.o of the voltage-controlled oscillator 11 varies with the value .DELTA..omega. as is clear from the equation (3) and the relation between them is as indicated by the solid line in FIG. 2.
It is known in the art that when the free-running frequency of the voltage-controlled oscillator is different from the frequency of the input signal e.sub.i, the output of the voltage-controlled oscillator 11 is locked to follow the input signal frequency but that the phase thereof is locked with a predetermined amount of shift .DELTA..PHI..sub.e from the phase of the input signal e.sub.i. Therefore, although it is desired to obtain as the output of the voltage-controlled oscillator a signal e.sub.o whose phase is shifted exactly by 90.degree. from that of the input signal e.sub.i, the phase is actually shifted by some value of 90.degree..+-..alpha.. Thus, correct quadrature demodulation cannot be achieved.
Accordingly, an object of this invention is to provide a phase control device in which control is effected so that the phase of the output signal of the voltage-controlled oscillator in the PLL circuit is shifted by a constant predetermined amount (90.degree.) from that of the input signal.